This equation computes the angle, `phi`of a vector from the magnitude of the angle, |`vecv`| and the x-component of the vector, `v_x`. See Figure B.
Young, Hugh and Freeman, Roger. University Physics With Modern Physics. Addison-Wesley, 2008. 12th Edition, (ISBN-13: 978-0321500625 ISBN-10: 0321500628 ) Pg 16 eq 1.6